[dm-devel] can we fix some dm snapshot crashes please? :)

[Mike Snitzer]

On Fri, May 07 2021 at 10:10P -0400,
Michael Tokarev <mjt at tls.msk.ru> wrote:

> 07.05.2021 15:31, Zdenek Kabelac wrote:
> > Dne 07. 05. 21 v 12:31 Michael Tokarev napsal(a):
> ...
> > >   sz=$(blockdev --getsize /dev/loop0)
> > >   dmsetup create base --table "0 $sz snapshot-origin /dev/loop0"
> > >   # and now the crash
> > >   mkfs.ext4 /dev/mapper/base
> 
> > Yes reproducible - can you please open BZ report here:
> > 
> > https://bugzilla.redhat.com/enter_bug.cgi?product=LVM%20and%20device-mapper
> 
> Ok, will do, thanks!

Thanks.  But when you do, please tone down the lamenting of how slow
snapshot crashes have been fixed -- or don't.  But that line of
rhetoric shows you're full of it.

> > I assume you are aware you are trying to using snapshot target in wrong way,
> > but it should not be crashing kernel.
> 
> Actually I'm not aware about the wrong way. The documentation is a bit lacking
> in this area. There are a few (maybe 2 or 3) guides describing the steps to do,
> most involves first using an 1:1 linear table, next replacing it on-the-fly with
> snapshot-origin table, but nowhere it is told that I can't create snapshot-origin
> directly, or that I need at least one snapshot device, or that the origin of the
> snapshot-origin table should be a dm device, or many many other variations.
> 
> I just tried to create a snapshot to be able to revert changes if something goes
> wrong, and since I don't use lvm I thought that plain dmsetup should do, but
> discovered that it doesn't quite work and it actually is a dangerous thing to use.. :)

Yes, it is dangerous to stab in the dark like you clearly are doing.